In specific optical arrangements, for example in extreme ultraviolet (EUV) lithography systems, it is necessary to arrange at least one portion of the beam path and thus also at least one portion of the optical elements in a vacuum environment. Such a vacuum environment can typically have a (total) pressure of between approximately 10−9 mbar and approximately 10−1 mbar in EUV lithography systems.
Within the meaning of this application, an EUV lithography system is understood to be an optical system for EUV lithography, i.e. an optical system which can be used in the field of EUV lithography. Alongside an EUV lithography apparatus that serves for producing semiconductor components, the optical system can be, for example, an inspection system for inspecting a photomask (also called reticle hereinafter) used in an EUV lithography apparatus, or for inspecting a semiconductor substrate (also called wafer hereinafter) to be structured, or a metrology system that is used for measuring an EUV lithography apparatus or parts thereof, for example for measuring a projection system.
In an EUV lithography system, but also in other optical arrangements, the presence of contaminating substances or particles in the vacuum environment or in the residual gas atmosphere present therein cannot be completely avoided. The contaminating substances can be polymers, for example, which originate from vacuum pumps or which are outgassed from adhesives. The contaminating substances can also be residues of photoresists applied on the wafer which are outgassed from the photoresist under the influence of the operating radiation and which can lead to carbon contaminations on the optical elements of the EUV lithography system or on other components in the vacuum environment.
It is known to remove contaminating substances or particles from optical surfaces with the aid of one gas nozzle or a plurality of gas nozzles, as is described for example in WO 2009/059614 A1 in the name of the applicant. For this purpose, the gas nozzle is aligned with the surface to be cleaned, and the surface to be cleaned is brought into contact with a gas flow of a cleaning gas, e.g. in the form of activated hydrogen or in the form of hydrogen radicals.
The efficiency of such cleaning depends on how strongly the contaminating particles adhere to the surface to be cleaned, i.e. the strength of the adhesion of the particles to the surface. Generally, in a vacuum environment, surfaces should be avoided which foster particle deposits, in particular surfaces having a high roughness, for which reason components having surfaces composed of electropolished high-grade steel or aluminium having very low roughness are often used in vacuum environments.
DE 10 2009 044 462 A1 discloses an optical element for filtering electromagnetic radiation, said optical element having a multilayer structure designed for reflecting EUV radiation. The optical element also has a grating structure designed for diffracting radiation in the visible to infrared wavelength range. In one example, the grating structure is designed for the destructive interference of radiation having an infrared wavelength of e.g. 10.6 μm. On the grating structure, it is possible to arrange an additional grating structure having a smaller grating constant and depth, which generates a destructive interference of radiation at least one further wavelength that is significantly shorter than the wavelength of the radiation that is filtered by the grating structure.